1. Field of the Invention
The present invention relates to a semiconductor device including a capacitor used in a dynamic random access memory (DRAM) or the like, and a method of manufacturing the same.
2. Description of the Background Art
A semiconductor device provided with a DRAM having a capacitor for storing signal information as charges has been used conventionally. Hereinafter, a structure of such a conventional semiconductor device provided with a DRAM having a capacitor will be described with reference to FIG. 18. In the conventional semiconductor device including the DRAM with the capacitor, as shown in FIG. 18, a source/drain region 101 is formed below a main surface of a semiconductor substrate 120 with a prescribed depth. An interlayer oxide film 102 is formed to cover semiconductor substrate 120 including source/drain region 101. A contact hole is formed which penetrates interlayer oxide film 102 to reach source/drain region 101. A storage electrode consisting of a polycrystalline silicon (hereinafter, referred to as xe2x80x9cpolysiliconxe2x80x9d) film 103 with an n type impurity introduced therein is formed to fill in the contact hole as well as to continuously cover the upper surface of interlayer oxide film 102. A capacitor dielectric film consisting of a tantalum oxide film 104, expressed as a chemical formula Ta2O5, is formed to continuously cover the surface of polysilicon film 103 as well as a portion of the upper surface of interlayer oxide film 102. A cell plate electrode consisting of a titanium nitride film 105, expressed as TiN, is formed by chemical vapor deposition (CVD) to cover the upper surface of tantalum oxide film 104.
In the conventional semiconductor device as described above, if a negative bias voltage is applied to the cell plate electrode, electrons in titanium nitride film 105 being the cell plate electrode are introduced into tantalum oxide film 104 being the capacitor dielectric film, whereby a leakage current is generated. This introduction of electrons from the cell plate electrode to the capacitor dielectric film causing the leakage current occurs when energy needed for electrons to exceed a potential barrier, which is determined by a work function of titanium nitride film 105 forming the cell plate electrode, is provided to the electrons within titanium nitride film 105. The fact that the leakage current is generated when the titanium nitride film, the work function of which is 4.95 eV, is used as the electrode means that a material having a work function that is greater than 4.95 eV needs to be used to form the electrode.
Further, in the manufacturing method of the conventional semiconductor device as described above, the step of forming tantalum oxide film 104 being the capacitor dielectric film is followed by the step of depositing titanium nitride film 105 that is to be the cell plate electrode. In this step of depositing titanium nitride film 105, ammonia (NH3) gas causing reduction is used, and thus, oxygen atoms that are components of tantalum oxide film 104 are eliminated. This causes a deficiency of oxygen atoms in tantalum oxide film 104, which in turn causes generation of a leakage current in the capacitor dielectric film.
The amount of charges stored in the capacitor reduces over time, due to the leakage current generated in the capacitor as described above. As a result, the charge retaining capability of the capacitor is diminished, which results in the degradation of refresh performance of the capacitor.
An object of the present invention is to provide a semiconductor device provided with a capacitor with an improved charge retaining capability, by suppressing generation of a leakage current in the capacitor dielectric film.
The semiconductor device according to a first aspect of the present invention includes: a contact plug including a tungsten film in the upper portion thereof, formed on a semiconductor substrate; a storage electrode including a tantalum nitride film formed on and contacting an upper surface of the tungsten film; a capacitor dielectric film including a tantalum oxide film formed on and contacting an upper surface of the tantalum nitride film; and a cell plate electrode including a tantalum nitride film formed on and contacting an upper surface of the tantalum oxide film.
In such a structure, the storage electrode and the cell plate electrode are formed of tantalum nitride film, the work function of which is greater than that of the titanium nitride film used to form the cell plate electrode of the capacitor in the conventional semiconductor device. This restricts introduction of electrons into the tantalum oxide film forming the capacitor dielectric film. Thus, it is possible to suppress generation of a leakage current in the capacitor dielectric film.
Further, the contact plug is formed using the tungsten film. Therefore, it is possible to prevent oxidation of the upper surface of the contact plug, which would be inevitable during a manufacturing process when a polysilicon film is used to form the storage electrode as in the case of the conventional semiconductor device. This prevents formation of additional capacitance because of the oxidation of the upper surface of the contact plug. As a result, reduction of capacitance of the capacitor is suppressed.
Moreover, the tantalum nitride film offering an effective barrier is formed on the tungsten film. Thus, counter diffusion between the tungsten film and the tantalum nitride film is prevented. This restricts generation of a leakage current in the capacitor dielectric film due to the counter diffusion between the contact plug and the storage electrode. As a result, the charge retaining capability of the capacitor is improved.
The semiconductor device according to a second aspect includes: a storage electrode including a tantalum nitride film formed on a semiconductor substrate; a capacitor dielectric film including a tantalum oxide film formed on and contacting an upper surface of the tantalum nitride film; and a cell pate electrode including a tantalum nitride film formed on and contacting an upper surface of the tantalum oxide film and a copper film formed on and contacting an upper surface of the tantalum nitride film.
In such a structure, the copper film used for the cell plate electrode is highly conductive, which increases drift speed of electrons within the cell plate electrode. Therefore, a capacitor can respond to the signal charges given to the capacitor at a greater response speed. As a result, the operating speed of the semiconductor device is increased. In addition, a part of the cell plate electrode is formed of the tantalum nitride film offering a good barrier, which prevents counter diffusion between the tantalum nitride film and the copper film. Thus, generation of a leakage current in the capacitor dielectric film due to the counter diffusion within the cell plate electrode is restricted. As a result, it is again possible to improve the charge retaining capability of the capacitor when the copper film is used as a portion of the cell plate electrode to increase the conductivity of the cell plate electrode.
The semiconductor device according to a third aspect of the present invention includes: a storage electrode including an indium oxide film formed on a semiconductor substrate; a capacitor dielectric film including a tantalum oxide film formed on and contacting an upper surface of the indium oxide film; and a cell plate electrode including an indium oxide film formed on and contacting an upper surface of the tantalum oxide film.
In such a structure, the storage electrode and the cell plate electrode are formed of indium oxide film, which minimizes the likelihood of a reductive elimination reaction of the tantalum oxide film forming the capacitor dielectric film occurring during the manufacturing process. Accordingly, possibility of elimination of oxygen atoms within the tantalum oxide film due to the reductive reaction is reduced. Thus, a high degree of crystallinity is kept for the tantalum oxide film constituting the capacitor dielectric film. This restricts generation of a leakage current in the capacitor dielectric film due to the deficiency of the oxygen atoms within the tantalum oxide film. As a result, it is possible to increase the charge retaining capability of the capacitor.
Preferably, in the semiconductor device according to the third aspect of the present invention, the storage electrode includes a tantalum nitride film formed beneath and contacting the lower surface of the indium oxide film.
In such a structure, the tantalum oxide film formed beneath and contacting the lower surface of the indium oxide film offers an effective barrier, which restricts counter diffusion between the indium oxide film and a conductive material used for the contact plug. Thus, it is possible to restrict generation of a leakage current in the capacitor dielectric film due to the counter diffusion between the indium oxide film and the conductive material used for the contact plug. As a result, by forming the storage electrode using indium oxide film that is unlikely to cause reductive reaction of the tantalum oxide film during the manufacturing process, it is again possible to improve the charge retaining capability of the capacitor.
More preferably, in the semiconductor device according to the third aspect of the present invention, the cell plate electrode includes a copper film formed on the indium oxide film.
In such a structure, the highly conductive copper film is used to form a part of the cell plate electrode. Thus, drift speed of the electrons within the cell plate electrode increases, and accordingly, the capacitor can respond to the signal charges given to the capacitor at a higher speed. As a result, the operating speed of the semiconductor device is increased.
Still more preferably, in the semiconductor device according to the third aspect of the present invention, in the case where the cell plate electrode includes the copper film on the indium oxide film, the cell plate electrode includes a tantalum nitride film formed between the indium oxide film and the copper film, contacting the both films.
In such a structure, the tantalum nitride film is formed between the two layers of the cell plate electrode, i.e., the indium oxide film and the copper film. This restricts counter diffusion between the indium oxide film and the copper film, and thus, generation of a leakage current in the capacitor dielectric film due to the counter diffusion within the cell plate electrode is restricted. As a result, it is possible to improve the charge retaining capability of the capacitor when the copper film is used as a part of the cell plate electrode to increase the operating speed of the semiconductor device.
The manufacturing method of the semiconductor device according to the first aspect of the present invention includes the steps of: forming a contact plug including a tungsten film on a semiconductor substrate; forming a storage electrode including a tantalum nitride film contacting an upper surface of the tungsten film; forming a capacitor dielectric film including a tantalum oxide film contacting an upper surface of the tantalum nitride film; and forming a cell plate electrode including a tantalum nitride film contacting an upper surface of the tantalum oxide film.
With such a manufacturing method, the storage electrode and the cell plate electrode are formed using tantalum nitride film, the work function of which is higher than that of the titanium nitride film that is used when manufacturing the cell plate electrode of the capacitor in the conventional semiconductor device. Therefore, a capacitor can be formed in which it is less likely that electrons are introduced into the tantalum oxide film forming the capacitor dielectric film. Thus, it is possible to manufacture a semiconductor device provided with a capacitor in which a leakage current is less likely to be generated in the capacitor dielectric film.
Further, the tungsten film is used to form a part of the contact plug. This prevents oxidation of the upper surface of the contact plug, which would be inevitable during manufacturing the storage electrode using the polysilicon film, as in the case of the conventional semiconductor device. Therefore, a capacitor can be formed in which formation of an additional capacitance due to the oxidation of the upper surface of the contact plug is prevented. As a result, it is possible to manufacture a semiconductor device in which reduction of the capacitance of the capacitor is restricted.
Moreover, the tantalum nitride film is formed on the tungsten film, which offers a good barrier. Thus, counter diffusion between the tungsten film and the tantalum nitride film is less likely to occur. As a result, a semiconductor device can be formed in which generation of a leakage current in the capacitor dielectric film due to the counter diffusion of the contact plug and the storage electrode is suppressed. Accordingly, it is possible to manufacture a semiconductor device provided with a capacitor with an improved charge retaining capability.
The manufacturing method of the semiconductor device according to the second aspect of the present invention includes the steps of: forming a storage electrode including a tantalum nitride film on a semiconductor substrate; forming a capacitor dielectric film including a tantalum oxide film contacting an upper surface of the tantalum nitride film; and forming a cell plate electrode by first forming a tantalum nitride film contacting an upper surface of the tantalum oxide film and then forming a copper film contacting an upper surface of the tantalum nitride.
With such a manufacturing method, the cell plate electrode is formed using the highly conductive copper film. This enables formation of a capacitor in which electrons within the cell plate electrode move at a high speed. Accordingly, the capacitor is allowed to respond to the signal charges given to the capacitor with a high response speed. Thus, it is possible to manufacture a semiconductor device provided with a capacitor operating at a high speed.
Further, the tantalum nitride film is formed as a part of the cell plate electrode, which offers a good barrier. Thus, a cell plate electrode can be formed in which counter diffusion between the tantalum nitride film and the copper film is less likely to occur. Therefore, a capacitor can be formed in which generation of a leakage current in the capacitor dielectric film due to the counter diffusion within the cell plate electrode is suppressed. As a result, it is possible to manufacture a semiconductor device provided with a capacitor with an improved charge retaining capability when a copper film is used as a portion of a cell plate electrode to increase conductivity of the cell plate electrode.
The manufacturing method of the semiconductor device according to the third aspect of the present invention includes the steps of: forming a storage electrode including a indium oxide film on a semiconductor substrate; forming a capacitor dielectric film including a tantalum oxide film contacting an upper surface of the indium oxide film; and forming a cell plate electrode including an indium oxide film contacting an upper surface of the tantalum oxide film.
With such a manufacturing method, as the indium oxide film is used, reductive reaction of the tantalum oxide film forming the capacitor dielectric film is unlikely to occur during the manufacturing process. Thus, possibility of elimination of oxygen atoms within the tantalum oxide film due to the reductive reaction is minimized. Therefore, a capacitor can be formed in which the tantalum oxide film constituting the capacitor dielectric film keeps a high degree of crystallinity, and thus, it is possible to form a capacitor in which generation of a leakage current in the capacitor dielectric film due to the deficiency of oxygen atoms within the tantalum oxide film is restricted. As a result, it is possible to manufacture a semiconductor device provided with a capacitor with an improved charge retaining capability.
Preferably, in the manufacturing method of the semiconductor device according to the third aspect of the present invention, the step of forming the storage electrode includes the step of forming a tantalum nitride film and the step of forming an indium oxide film contacting an upper surface of the tantalum nitride film.
With such a manufacturing method, the tantalum nitride film offering an effective barrier is formed beneath and contacting the lower surface of the indium oxide film forming the storage electrode. Therefore, a capacitor can be formed in which counter diffusion between the indium oxide film and a conductive material used for the contact plug is suppressed. This allows formation of a capacitor in which generation of a leakage current in the capacitor dielectric film due to the counter diffusion between the indium oxide film and the conductive material used for the contact plug is suppressed. As a result, it is possible to manufacture a semiconductor device provided with a capacitor with an improved charge retaining capability when the storage electrode is formed using the indium oxide film which is unlikely to cause reduction of the tantalum oxide film during the manufacturing process.
More preferably, in the manufacturing method of the semiconductor device according to the third aspect of the present invention, the step of forming the cell plate electrode includes the step of forming an indium oxide film and the step of forming a copper film on the indium oxide film.
With such a manufacturing method, the highly conductive copper film is formed as a part of the cell plate electrode, which allows formation of a capacitor in which electrons within the cell plate electrode move at a high speed. This increases the response speed of the capacitor when it responds to the signal charges given to the capacitor. It is thus possible to manufacture a semiconductor device which operates at a high speed.
Still more preferably, in the manufacturing method of the semiconductor device according to the third aspect of the present invention, in the case where the step of forming the cell plate electrode includes the step of forming the indium oxide film and the step of forming the copper film on the indium oxide film, the step includes the step of forming a tantalum nitride film contacting an upper surface of the indium oxide film, after the formation of the indium oxide film and before the formation of the copper film.
With such a manufacturing method, the tantalum nitride film a offering a good barrier is formed between two layers of the cell plate electrode, i.e., the indium oxide film and the copper film. This allows formation of a cell plate electrode in which counter diffusion between the indium oxide film and the copper film is restricted. Thus, a capacitor can be formed in which generation of a leakage current in the capacitor dielectric film due to the counter diffusion of the indium oxide film and the copper film is suppressed. As a result, it is possible to manufacture a semiconductor device provided with a capacitor with an improved charge retaining capability when a copper film is used as a part of the cell plate electrode to form a capacitor having a high operating speed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.